Human cytomegalovirus (CMV) infection has been associated with several neoplastic diseases, including prostatic carcinoma, adenocarcinoma of the colon, cervical carcinoma, and Kaposi's sarcoma. The morphological transforming region II (mtrII) of CMV Towne has been localized to a 980 base-pair fragment containing three putative open reading frames (ORFs) of 79, 83, and 34 amino acids (aa). Homologous regions exist in strains Towne, AD169 and Tanaka. However, Tanaka mtrII has reduced transforming activity when compared to Towne and AD169. A 79 aa (ORF) present in both Towne and AD169 is truncated in Tanaka, indicating that it may be crucial for efficient transformation. The promoter activity of the 980 bp mtrII DNA fragment has been investigated. The experiments showed that the 980 bp colinear DNA acts as a promoter when inserted in the sense orientation with reference to the ORF but not in the opposite orientation. In further experiments, two distinct promoter regions were detected by separately testing the left-hand 440 bp and the right-hand 540 bp fragment. Promoter activity was shown to be similar in all three strains, implying that this is not a factor in transformation, thus emphasizing the importance of the 79 aa ORF.